1. Field of the Invention
This invention relates to an image forming apparatus such as copying machine, printer, facsimile, etc. equipped with a fixing device to heat, press and fix unfixed toner images or multi-functional image forming apparatus commonly provided with these image forming units and an image forming method.
2. Description of the Related Art
As a fixing device that is used in electro-photographic copy machine, printer, facsimile or multi-functional peripheral (hereinafter abbreviated to MFP), there is a fixing device to heat, press and fix unfixed toner images by inserting a recording medium between fixing roller pair comprising a heating roller and a press roller or similar fixing belts.
In such a fixing device, when a fixing temperature and pressing force are always at constant levels, fixation may differ depending upon such characteristics as thickness, shape or material of recording media. When heating and pressing to a recording medium are excessive at the time of fixing, the recording medium may shrink or wrinkle and on the other hand, when heating and pressing are insufficient, the fixing become defective and deterioration of image quality resulted.
Therefore, a heating pressure fixing device is disclosed in Japanese Patent Application No. 2000-19886. This fixing device is capable of switching a fixing temperature, a pressing force between fixing rollers or both of them to optimum levels according to moisture content in recording media.
In the conventional fixing roller device described above, specific operations when changing fixing conditions according to moisture contents in recording media are not disclosed. However, in similar apparatus, while fixing temperatures are changed through a control panel, pressing force was generally switched by changing the position of the lever of a pressure mechanism provided to fixing rollers. In other words, to change a fixing temperature and pressure force of a heating pressure fixing device to optimum levels, it was so far necessary for operator to execute two operates; not only the operation through the control panel but also the lever operation of a pressure mechanism.
So, an image forming apparatus capable of making easy to operate when changing fixing temperature and pressure force to get fixed images of good quality by a heating pressure fixing device and easily obtaining a good fixing is desired.